The invention relates to an integrated optical element on a substrate on InP which element comprises a waveguiding layer of a specific, complex refractive index being interposed between two layers of semiconductor material having real refractive indices with one of the two layers being on the substrate, the complex refractive index of the waveguiding is controllable with electric charge carriers and exhibits a real part that is greater than the real refractive index of the two layers so that the light of a specific wavelength can be coupled into the waveguide layer and guided therein and the waveguiding layer exhibits a crystal lattice constant that is less than one percent greater than a specific lattice constant of the InP and the element includes means for controlling the complex refractive index of the waveguiding layer so that the intensity and/or phase of the light being guided in the waveguiding layer is variable.
An integrated optical element of said species is disclosed by the document GB-A-2 207 283 in which five different examples of integrated optical elements on a substrate of InP are described.
What all of these examples have in common is that the layers of ternary material adjoining the waveguiding layer are composed of such a composition that these layers exhibit the same crystal lattice constant as the InP of the substrate.
In two examples, the waveguiding layer is composed of three adjacent, thin layers at which the layers adjacent to the waveguiding layer adjoin. These outer, thin layers each respectively exhibit a crystal lattice constant that gradually increases in the direction toward the central thin layer from the relatively lower crystal lattice constant of the layers adjacent to the waveguiding layer to the relatively higher crystal lattice constant of the central, thin layers.
In one of these two latter examples, the relatively higher crystal lattice constant of the central thin layer is only 0.5% higher than the crystal lattice constant of the InP of the substrate.
An integrated optical element on a substrate of InP composed of
a waveguiding layer of a specific, complex refractive index and
two layers of semiconductor material each respectively exhibiting a real refractive index between which the waveguiding layer is arranged, that adjoin the waveguiding layer and whereof one is located between the waveguiding layer and the substrate, whereby
the complex refractive index of the waveguiding layer is controllable with electrical charge carriers and exhibits a real part that is respectively greater than the real refractive index of the two layers adjoining the waveguiding layer, whereby
light of a specific wavelength can be coupled into the waveguiding layer and the in-coupled light is guided essentially in the waveguiding layer, and whereby
a means is provided for the control of the complex refractive index in the waveguiding layer such that the intensity and/or phase of the light guided in the waveguiding layer is variable, proceeds from the following documents:
a) K. Magari et al.: "Polarization-insensitive optical amplifier with tensile-strained-barrier MQW structure", IEEE J. Quantum Electr., Vol. Qe-30, No. 3 (1994) pp. 695-702, from
b) M. A. Newkirk et al.: "1.5 .mu.m Multi-quantum-well semiconductor optical amplifier with tensile and compressively strained wells for polarization-independent gain", IEEE Photon. Technol. Lett., Vol. PTL-4, No. 4 , 1993, pp. 406-408, from
c) P. J. A. Thijs et al.: "Progress in long-wavelength strained-layer InGaAs (P) quantum-well semiconductor lasers and amplifiers", IEEE J. Quantum elektron. Vol. QE-30, No. 2 (1994), pp. 477-499 from
d) Ch. Holtmann et al "Polarization-insensitive bulk ridge-type semiconductor optical amplifiers at 1.3 .mu.m wavelength", Yokohama, Japan, Jul. 4-6, 1993, Paper Sub 2-1, pp. 8-11 (1993) or also from
e) Tiemeijer et al.: "High Performance 1300 nm Polarisation insensitive laser amplifiers employing both tensile and compressively strained quantum wells in a single active layer" Proc. Europ. Conf. Opt. Commun. '92 (ECOC'92), Pt. 3, Berlin, 1992, pp. 911.
Each of the waveguides described in these documents a) through e) forms a waveguide in the form of a polarization-independent optical amplifier.
In the case of the amplifier described in document a), the wave-guiding layer has quantum well layers in the form of weakly tensile-strained potential wells or potential barriers. Wealky tensile-strained wells require relatively high current because of "valence band mixing". Quantum wells with tensile-strained barriers require large thicknesses, which are difficult to manufacture because of the strain and which move structures into the range of conventional layered structures without a quantization effect.
In the case of the waveguides described in document b) and c), the wave-guiding layer has quantum well layers in the form of potential wells which are alternately tensile-strained and compression-strained. Such quantum well layers have been successfully implemented in optical amplifiers for a wavelength of 1.3 .mu.m.
The waveguide described in document d) is a ribbed waveguide whose wave-guiding layer has a homogenous composition of InGaAsP with a gap wavelength of 1.3 .mu.m and a thickness between 150 nm and 400 nm.
The waveguide known from document e) is a buried heterostructure waveguide with polarization-independent waveguidance.
The document EP-A-612 129 discloses an integrated optical element on an n-doped substrate of GaAs, whereby a waveguiding layer is arranged between two layers of semiconductor material that adjoin the waveguiding layer and whereof one layer is located between the waveguiding layer and the substrate and is n-doped, whereas the other layer is p-doped.
In this element, the waveguiding layer generally exhibits a crystal lattice constant that is smaller than a specific crystal lattice constant of the GaAs of the substrate.
One exemplary embodiment of this element has the characteristic that each of the layers adjoining the waveguiding layer, whereof one layer is located between the waveguiding layer and the substrate, respectively exhibit a crystal lattice constant that is greater than the crystal lattice constant of the GaAs of the substrate as well as greater than the crystal lattice constant of the waveguiding layer, and that a carrier layer adjoining the substrate and referred to in this document as a buffer layer is arranged between the one layer located between the waveguiding layer and the substrate and adjoining the waveguiding layer and the substrate, said buffer layer being composed of semiconductor material with a crystal lattice constant that, at the substrate, is the same as the specific crystal lattice constant of the GaAs of the substrate and increases from the substrate in the direction toward the one layer adjoining the waveguiding layer, increasing to the greater crystal lattice constant of this one layer. The greater crystal lattice constant is approximately 1% greater than the crystal lattice constant of the substrate.
The carrier layer is separated from the layer adjoining the waveguiding layer by a layer stack that is referred to as super lattice layer in this document, this being composed of two types of extremely thin layers of semiconductor material having a different band gap. The crystal lattice constant of the layer stack is matched to that of the part of the carrier layer lying closest.